time zone general names: upper case for constant

src/corelibs/string_handling/timestamp_strings.py

@@ -11,11 +11,11 @@ class TimestampStrings:
     set default time stamps
     """
 
-    time_zone: str = 'Asia/Tokyo'
+    TIME_ZONE: str = 'Asia/Tokyo'
 
     def __init__(self, time_zone: str | None = None):
         self.timestamp_now = datetime.now()
-        self.time_zone = time_zone if time_zone is not None else self.time_zone
+        self.time_zone = time_zone if time_zone is not None else self.TIME_ZONE
         try:
             self.timestamp_now_tz = datetime.now(ZoneInfo(self.time_zone))
         except ZoneInfoNotFoundError as e: